Electrical energy storage systems are currently used for powering portable electronic devices (such as phones and laptops), but they are increasingly required for future large battery applications, such as for plug-in hybrid vehicles/electric vehicles and for the storage of energy generated by the wind, the Sun, and nuclear fusion. Because rechargeable (secondary) lithium and lithium ion batteries have some of the highest energy storage capabilities, there has been extensive research to improve their energy density, power density, and safety, as risks increase with the size of the energy-storage device. Failure mechanisms and safety hazards in lithium and lithium ion batteries arise as a result of the development of shorts between the anode and cathode after many charge/discharge cycles, as the result of lithium dendrite formation (when lithium metal is used) and the presence of both a combustible material and an oxidizing agent, which can result runaway reactions and fires or explosions.
Demand for safe, high capacity electrical energy storage devices has motivated the development of solid polymer electrolytes (SPEs) that are compatible with lithium metal, and thus could utilize its high specific capacity (3860 mA h g−1). SPEs, of which the most investigated has been polyethylene oxide (PEO), are flexible compared with inorganic solid electrolytes, and do not suffer from safety issues such as leakage, shorts due to dendrite formation, and explosions due to volatile solvents that occur in the liquid electrolytes currently used in lithium ion batteries; further, they have longer cycle life due to the slower migration of degradation products in solid compared with liquid electrolytes to reactive centers in the electrodes. SPEs are critical components on the anode side in cathode flow batteries. However, SPEs have lower ambient temperature ionic conductivities, σ, (σ<10−5 S/cm) than either liquid or gel electrolytes.
Attempts to improve the ambient and sub-ambient temperature conductivity (σ) of solid polymer electrolytes often have yielded materials with poor mechanical properties. Single ion conductors, in which the anion is immobile, have even lower room temperature (RT) ionic conductivities (<10−6 S/cm), but have lithium ion transference numbers, tLi+, the fraction of the charge carried by Li+, that approach 1, so that in principle all of the conductivity, although low, originates from the migration of the electroactive lithium species (M. Doyle et al., Electrochimica Acta 1994, 39, 2073; Thomas et al., Journal of Power Sources 2000, 89, 132). By contrast, lithium ion transference numbers for PEO electrolytes with mono-ionic lithium salts (LiX), where X is the anion, are typically tLi+ in the range 0.2-0.3 (Gray, Solid Polymer Electrolytes-Fundamentals and Technical Applications, VCH, Wenheim 1991; Shin et al., Journal of the Electrochemical Society 2005, 152, A283; Stephan et al., Journal of Physical Chemistry B 2009, 113, 1963).
Previous attempts to improve conductivity (σ), interfacial and transport properties of PEO, which have included the addition of plasticizers (Kim et al., Solid State Ionics 2002, 149, 29) and nanoparticle fillers such ceramic ZrO2, SiO2 (Kim et al., Electrochimica Acta 2007, 52, 3477), Al2O3 (Croce et al., Nature 1998, 394, 456; Croce et al., Journal of Physical Chemistry B 1999, 103, 10632), chitin (Stephan et al. et al., Journal of Physical Chemistry B 2009, 113, 1963) and polyphosphazine (Zhang et al., Electrochimica Acta 2011, 55, 5966). However, comprehensive evaluation showed minimal improvement in conductivity (Syzdek et al., Electrochimica Acta 2010, 55, 1314). Conduction in PEO based electrolytes occurs predominantly in the amorphous phase, but amorphous PEO, even with added salt or fillers, is a viscous liquid. Thus, preparation of SPEs/SICs from PEO has consisted of engineering a two phase morphology in which there is both a structural and a conductive phase, either through block copolymers, or polymers with pendant oligomeric polyethylene glycols (PEGs). However, conductivity has been shown to increase in two phase morphologies for higher molecular weight PEOs (Gomez et al., Nano Letters 2009, 9, 1212; Panday et al., Macromolecules 2009, 42, 4632.
Chinnam and Wunder, Chemistry of Materials 2011, 23, 5111, describe mixtures of POSS-PEG8, a polyoctahedral silsesquioxane functionalized with eight PEG, —(CH2CH2O)m—, chains (m˜13.3) and the multi-ionic lithium salt, POSS-phenyl7(BF3Li)3, made by reaction of POSS-phenyl7Li3 with BF3(OC2H5)2. POSS-phenyl7(BF3Li)3 has Janus-like properties, with one end predominantly hydrophobic and the other end ionic in character. These blends, for which the PEG crystallized/melted below 0° C., exhibited a crystallization exotherm above 50° C., which was attributed to aggregation of the phenyl groups of POSS-phenyl7(BF3Li)3. However, these materials remained viscous liquids unless the amount of POSS-phenyl7(BF3Li)3 was so high that conductivity drastically decreased, as the PEG phase became discontinuous.
What is needed is an alternative approach to engineering solid polymer electrolytes with good mechanical stability as well as high ionic conductivities and high lithium ion transference.